1. Field of the Invention
The present invention relates to an output module equipped with an ON/OFF function for output common, and particularly relates to an output module for programmable controller to control the sequence of a machine.
2. Description of the Related Art
A programmable controller is a device to control the operation of a NC machine tool in accordance with predetermined sequence. The programmable controller includes a memory that stores instructions to execute various functions, including sequence, timing and operational expression, and is built in a numerical control device. A sequence program for the programmable controller is to create the procedure of operation to be performed by a CPU in the form of a ladder diagram, and is converted into a machine language instruction by a Programmable Machine Controller (PMC) programmer equipped with a program creation function and is then stored in the memory.
An output module for such a programmable controller includes: a control circuit that receives an instruction from a higher-level device and turns ON/OFF load output circuits; the load output circuits that receive an ON/OFF instruction from the control circuit to control current flowing through a load connected; and an output common that is externally connected to a power supply for load and supplies load current to each of the load output circuits. Each of the load output circuits includes a terminal connected to a corresponding load and a terminal connected to the power supply for load. While a plurality of the load output circuits are typically prepared to control a plurality of loads, the load output circuits share one common terminal in many cases that is connected to the power supply for load so as to reduce the number of terminals.
FIG. 8 schematically shows a conventional output module. Reference numeral 10 denotes an output module, which internally includes a plurality of load output circuits 40 (40a to 40n). Each load output circuit 40 includes an output terminal 50 that can be connected to an external load 42. The output module has an output common terminal 60 apart from the output terminals 50, and the output common terminal 60 can be connected to an external power supply for load 80. This output common terminal 60 is to reduce the number of terminals, and if power supplies for load are individually provided for the load output circuits 40, they will make the structure of the output module complicated. That is why they are connected as the output common terminal 60 collectively to the power supply for load 80 to reduce the number of terminals, so that the output common terminal 60 and each of the load output circuits 40 (40a to 40n) are connected to supply power to these load output circuits 40.
The output module 10 internally includes a control circuit 30 as well. The output module 10 is then connected to a higher-level device 70, from which an ON/OFF instruction of the load output circuits 40 is issued to the control circuit 30. Receiving such an instruction, the control circuit 30 then issues an ON/OFF instruction to each of the load output circuits 40.
An open/close mechanism 92 is disposed between the output common terminal 60 and the power supply for load 80, and the open/close mechanism 92 is connected to an external device 90, from which an ON/OFF control instruction is issued, whereby load current is allowed to flow or interrupted between the output common terminal 60 and the power supply for load 80.
In such an output module 10, if an abnormal situation or an unexpected control state happens, all of the load output circuits have to be turned OFF at the same time in some cases. Then, an instruction is issued from the higher-level device 70 to the control circuit 30 so as to turn OFF each of the load output circuits 40, and each of the load output circuits 40 stops the output in response to the instruction from the control circuit 30, or the open/close mechanism 92 of the output common terminal 60 interrupts the load current to the output common terminal 60, for example.
JP S63-88607 A discloses a technique relating to an output unit for sequence controller that is configured so that a signal from an external CPU unit is output to an output circuit via an interface.
The technique disclosed in JP S63-88607 A, however, has the following risk. That is, since it does not include any special device to interrupt the continuity between the external power supply for load and the output common terminal, if all of the load output circuits have to be turned OFF at the same time due to an abnormal situation or an unexpected control state as described for the conventional technique, an instruction for turning OFF has to be issued from the CPU unit to each of the output circuits, which may make a control program from the CPU complicated and may increase the capacity of the program.
In the conventional output module illustrated in FIG. 8, one method to turn OFF all of the load output circuits 40 at the same time includes to send an instruction from the higher-level device 70 to the control circuit 30 to turn OFF each of the load output circuits 40 so as to allow each of the load output circuits 40 to stop the output in response to the instruction from the control circuit 30. Similarly to the technique disclosed in JP S63-88607 A, this method also has the risk of making a control program from the higher-level device 70 complicated and increasing the capacity of the program.
As another means, load current to the output common terminal 60 may be interrupted by the open/close mechanism 92 of the output common terminal. However, this requires the external device 90 to open/close the output common terminal 60 and its control means to be provided separately.